Generally, most of printed wiring boards are produced by a method comprising: impregnating a thermosetting resin such as epoxy resin to a substrate such as glass cloth, then drying the substrate to make a prepreg; piling up single or multiple sheets of said prepreg as well as a copper foil if necessary and applying heat and pressure to make a laminate; and subsequently subjecting said laminate to a photolithography and etching treatments or plating to form a circuit pattern made of copper foil.
Further, a multilayer printed wiring board can be produced by a sequential forming method comprising: using the above-described printed wiring board as a core substrate; piling up a prepreg on a surface layer thereof, as well as further piling up a copper foil on the outside thereof; applying heat and pressure thereto to make a multilayer substrate; and subsequently forming a circuit on the surface of the multilayer substrate.
On the other hand, due to recent tendencies to high-functionality and reduction in size and weight of digital equipment, further reduction in size and thickness, as well as increase in density have also been required for the printed wiring board to be used therefor. As a means for that, increase in density is being achieved by thinning a glass cloth to be used as a substrate, as well as increasing the number of layers in a build-up multilayer printed wiring board by the sequential forming method. Under such circumstance, thickness of the glass cloth has been required to be thinned from 40 μm to 10 μm, due to the requirements of increasing in number of layers and thinning to the printed wiring board.
Generally, in the above-described production process for the printed wiring board, it has been known that dimensional change, as well as warpage and twist of a copper-clad laminate are generated by heat and pressure in the laminating step, and etching out of a part of copper foil in the circuit pattern forming step.
However, a thin glass cloth of 40 μm or less has a weak mechanical strength in the X-Y plane direction and a large anisotropy thereof compared with those of a thick glass cloth. In addition, bowed filling and yarn slippage tend to occur. For this reason, there was a problem that, in fabrication of a copper-clad laminate using a glass cloth of 40 μm or less, problems relating to the anisotropy in dimensional change, warpage and twist remarkably easily occur compared to a thick glass cloth.
As a method to enhance a mechanical strength in the X-Y plane direction and to improve an anisotropy thereof of the glass cloth, the following methods and the like have been proposed: (1) a method where reinforcement effects by stiffness of glass yarns are made equivalent by making amounts of glass to be filled, in the warp yarn direction and the weft yarn direction, equivalent, (2) a method where balance of reinforcement effects is improved by optimizing weaving density, crimp percentage, and the like, in the warp yarn direction and the weft yarn direction, (3) a method where restrictive property by yarn itself is enhanced by narrowing a distance between yarns by spreading processing.
(1) As a method where reinforcement effects by stiffness of glass yarns are made equivalent by making amounts of glass to be filled, in the warp yarn direction and the weft yarn direction, equivalent, such a method has been known that a woven fabric structure where the same kind of yarn is woven by the same weaving density both in the warp yarn direction and the weft yarn direction is employed, as disclosed in Patent Document 1 described below.
However, since a glass cloth is generally produced as a long cloth in a state where tension is applied in the warp direction, waving of the weft yarn to which tension is not applied tends to become more significant as compared with the warp yarn on which tension does not act, even if weaving is carried out using the same kind of yarns for the warp yarn and the weft yarn in the same weaving density. As a result, this causes an anisotropy in dimensional change. Also, when prepreg is produced, since resin is impregnated and dried in a state where tension acts on the warp yarn direction, and waving in the warp yarn direction is removed in some degree depending on a level of the tension, difference in waving states between the warp yarn direction and the weft yarn direction tends to become larger and difference in reinforcement effects tends to be expanded. Further, there is also the following problem. In the case of a thin glass cloth having a thickness of 40 μm or less, since glass yarn constituting the glass cloth has too small diameter and weak stiffness, influence on the anisotropy of the tension applied on the warp yarn direction in weaving and prepreg-making becomes larger together with thinning of the glass cloth, and the problem of anisotropy in dimensional change appears more significantly. Therefore, the method where amounts of glass to be filled in the warp yarn direction and the weft yarn direction are made equivalent was insufficient to improve the anisotropy in dimensional change, in particular, in the case of a thin glass cloth having a thickness of 40 μm or less.
(2) As a method where balance of reinforcement effects is improved by controlling weaving density and crimp percentage, and the like in the warp yarn direction and the weft yarn direction, the following Patent Document 2 discloses a glass cloth where a comparatively thick yarn is woven in the majority; the following Patent Documents 3 to 5 disclose a glass cloth where balance of weaving density and crimp percentage is optimized; the following Patent Document 6 discloses a glass cloth where cross-sectional shapes and waving states are made equivalent by spreading the warp yarn and the weft yarn in the same degrees; and the following Patent Documents 7 to 9 disclose a glass cloth configured with the weft yarn thicker than that of the warp yarn.
Patent Document 2 discloses a glass cloth having a high cloth weight where a thick yarn of 66 tex (filament diameter: 9 μm, filament number: 400) or more is woven as densely as 40 yarns/25 mm, improving the total reinforcement effect by increasing amount of glass.
However, since the glass cloth disclosed in Patent Document 2 weaves a thick yarn densely to obtain a high cloth weight, thickness of the glass cloth is as comparatively thick as 178 to 183 μm as disclosed in Examples 1 to 6 of Patent Document 2.
In the case of a thin glass cloth having a thickness of 40 μm or less, even when amount of glass is increased by increasing weaving density of glass yarn, it is difficult to improve dimensional stability and anisotropy thereof, because stiffness cannot be increased as much as in a thick glass cloth. In recent years, for aiming at improving stiffness of thin glass cloth having a thickness of 40 μm or less, increasing of weaving density of glass yarn have been often attempted, but it is a current situation that anisotropy of dimensional stability has not been improved yet.
Patent Documents 3 to 5 disclose a glass cloth where a yarn of filament diameter of 9 μm or 7 μm is used, distance between yarns in the warp yarn and the weft yarn is narrow, and weaving density and crimp percentage in the warp yarn direction and the weft yarn direction have been optimized, and disclose that variation by cure shrinkage of resin is reduced due to less resin amount present in the space between glass yarns, and reinforcement effects in the warp yarn direction and the weft yarn direction become equivalent because weaving shrinkage in the warp yarn direction and the weft yarn direction is set adequately.
However, thicknesses of the glass cloths in Patent Documents 3 to 5 are as comparatively thick as 90 μm or more as disclosed in Examples of these Patent Documents, because such a thick filament as filament diameter of 9 μm or 7 μm is used. Usually, when a thin glass cloth having a thickness of 40 μm or less is required, a thin filament having a filament diameter of 5 μm or less should be used. Since a thin filament having a filament diameter of 5 μm or less has a low stiffness, crimp percentage of the warp yarn tends to become smaller due to the tension exerted on the warp yarn direction in weaving, contrary a crimp percentage of the weft yarn tends to become larger, and it was difficult to optimize the crimp percentage so that reinforcement effects in the warp direction and the weft direction become equivalent.
Patent Document 6 discloses a glass cloth which is the one having a thickness of 10 μm or more but 50 μm or less, where the warp yarn and the weft yarn are configured with the same kind of yarn, and cross-sectional shapes and waving states of the warp yarn and the weft yarn are equivalent because spreading treatment is carried out under nearly a tensionless condition. It has been disclosed that in said glass cloth, a ratio of an elongation percentage of the warp yarn when a load in a range of 25 to 100 N per 25 mm width is applied in the warp yarn direction, relative to an elongation percentage of the weft yarn when said load is applied in the weft yarn direction, is 0.8 or more but 1.2 or less, and anisotropy in XY direction can be improved because elongation amounts of the warp yarn and the weft yarn are equivalent under a certain level of tensile stress.
However, even if spreading treatment is attempted in nearly a tensionless state, since not a low tension is exerted in the transporting direction (the warp yarn direction), it is difficult to make waving states of the warp yarn and the weft yarn equivalent. Even if elongation amounts of the warp yarn and the weft yarn under a tensile tension in a range of 25 to 100 N per 25 mm width can be made equivalent, it is still difficult to make the elongation percentages of the warp yarn direction and the weft yarn direction equivalent under a further lower load region (for example, 5 N), and it was not satisfactory with regard to improvement of the anisotropy in dimensional change. In addition, since physical force is exerted in nearly a tensionless state, that is, in a state where glass cloth is not held, there is also a problem that, in particular, in the case of a thin glass cloth, bowed filling and yarn slippage, and the like tend to occur, which cause warpage or twist of laminate.
Patent Document 7 discloses a glass cloth where monofilament diameter of the weft yarn is 9.5 μm or more and thicker than that of the warp yarn, weight ratio of weft yarn/warp yarn is 0.8 or more but 1.2 or less, and weight is 200 to 300 g. Patent Document 7 discloses that said glass cloth can minimize coordinate shift by using a yarn having a thick monofilament diameter, and the anisotropy can be solved without enlarging the coordinate shift by using the weft yarn having a thick monofilament diameter than that of the warp yarn.
In addition, Patent Document 8 describes a glass cloth where the warp yarn has a filament diameter of 9 and a filament number of 400, the weft yarn has a filament diameter of over 9 μm but 10.5 μm or less and a filament number of 400, a ratio of weaving density per 25 mm width of the warp yarn and the weft yarn is 1.0 to 1.4 or less, and a weight is 180 to 250 g. It has been shown that, in said glass cloth, occurrence of warpage or twist can be inhibited by using a thick yarn within the above-described range and making thicknesses of the warp yarn and the weft yarn different within the above-described range.
In addition, Patent Document 9 describes a glass cloth where count of the weft yarn is larger than that of the warp yarn, and a value of a product of count of the warp yarn and weaving density of the warp yarn divided by a product of count of the weft yarn and weaving density of the weft yarn is 0.8 or more but 1.2 or less. It has been disclosed that, in said glass cloth, a frequency of waving is reduced by increasing a count of the weft yarn, and hence reinforcement effects in the warp yarn direction and the weft yarn direction become nearly equivalent, and warpage and twist of a printed wiring board can be reduced, and that when a thick yarn of 60 tex or more is used for both of the warp yarn and the weft yarn, effect thereof becomes significant.
Patent Documents 7 to 9 disclose that the glass cloth configured with a thicker weft yarn than the warp yarn can improve dimensional stability as well as warpage and twist of a printed wiring board, but in any case, a thick yarn of 60 tex or more is used to enhance the reinforcement effect. Since glass cloth configured with a thick glass yarn tends to exhibit a large waving structure and a large elongation amount when a tensile stress is applied, dimensional change of a printed wiring board becomes significant, and therefore, it was not sufficient one in the point that variation and anisotropy in dimensional change tended to become large.
In addition, in the cases of the glass cloths described in Patent Documents 7 to 9, since a thick yarn of 60 tex or more is used as described above, thickness of a glass cloth becomes comparatively thick (glass cloths having a thickness of 188 μm in Examples 1 and 2 of Patent Document 7, 180 μm in Examples 1 to 3 of Patent Document 8, and 180 to 250 μm in Examples 1 to 5 of Patent Document 9 have been disclosed), and they were not the one which can improve dimensional stability of a thin glass cloth having a thickness of 10 to 40 μm, which was the purpose of the present application.
The glass cloths registered with ICP include, as the one in which the warp yarn is thicker than the weft yarn, 1651 (warp yarn: G 150, weft yarn: G 67, thickness: 135 μm), 2125 (warp yarn: E 225, weft yarn: G 150, thickness: 91 μm), 2157 (warp yarn: E 225, weft yarn: G 75, thickness: 130 μm), 2165 (warp yarn: E 225, weft yarn: G 150, thickness: 101 μm), 2166 (warp yarn: E 225, weft yarn: G 75, thickness: 140 μm), 7635 (warp yarn: G 75, weft yarn: G 50, thickness: 201 μm), 7642 (warp yarn: G 75, weft yarn: G 37, thickness: 254 μm), 1657 (warp yarn: G 150, weft yarn: G 67, thickness: 150 μm), 3133 (warp yarn: E 225, weft yarn: G 150, thickness: 81 μm), 3323 (warp yarn: DE 300, weft yarn: E 225, thickness: 86 μm), 7640 (warp yarn: G 75, weft yarn: G 50, thickness: 249 μm), 7669 (warp yarn: G 75, weft yarn: G 67, thickness: 178 μm), 7688 (warp yarn: G 75, weft yarn: G 67, thickness: 190 μm), 1165 (warp yarn: D 450, weft yarn: G 150, thickness: 101 μm), and 3132 (warp yarn: D 450, weft yarn: E 225, thickness: 71 μm), and all of these cloths are thick glass cloths which have a thickness of 70 μm or more using a thick filament having a diameter of 7 μm for the weft yarn. It has been known that when a glass yarn thicker than the warp yarn is used for the weft yarn, it becomes difficult to obtain a thin cloth; the cloth lacks surface smoothness; and further anisotropy in the reinforcement effects such as dimensional stability and coefficient of thermal expansion tends to occur due to different stiffness of glass yarn in warp yarn direction and weft yarn direction. It is present situation that the same glass yarns are usually used for the warp yarn and the weft yarn in a thin glass cloth having a thickness of 40 μm or less.
Besides the above, the glass cloth configured with yarns having different thicknesses for the warp yarn and the weft yarn are also disclosed in Patent Documents 10 to 15 described below, but none of the above glass cloths could not improve dimensional stability and anisotropy thereof.
(3) The glass cloth where restrictive property by yarn itself is enhanced by narrowing a distance between yarns by spreading processing has been disclosed in Patent Documents 16 to 19 described below. In the glass cloths disclosed in these Patent Literatures, an effect of the dimensional stability can be obtained because a gap between glass yarns is narrow and variation thereof is also small, and therefore cure shrinkage of the resin present in said gap is reduced, and contact area of the warp yarn and the weft yarn is large and hence resistance to the cure shrinkage is large.
However, since mechanical strength and elongation characteristics of the warp yarn and the weft yarn are influenced by stiffness of the yarn and waving state, sufficient effect for improving anisotropy in dimensional change rate could not be obtained only by narrowing the gap between yarns by spreading treatment.
As described above, a glass cloth having a thickness of 40 μm or less which allows to produce a printed wiring board having a small anisotropy in dimensional change and being free from warpage and twist with good accuracy, has not been obtained until now, and such glass cloth has been strongly demanded.